How do I balance tow chokes on the same Weber?

[Bruce Crowthorne]

At last, I now have the engine running in my car after a 20 month rebuild!

Got the ignition timing fixed out and have made an initial attempt at balancing the carbs.

However, chokes 3 and 4 (ie on the same carb) are out of balance.
Since a picture is worth a thousand words have a look at the output on my "Carbtune". Results using a Synchrometer show the same pattern ...

Question is how do I balance the two chokes?

I have read about drilling holes in a butterfly and in another book it suggests twisting the throttle spindle.

Who has actually used either of these approaches (or any other) and how effective was it?

Any advice gratefully received
Bruce

[Bruce Crowthorne]

Oops, I really meant two chokes
Bruce

[tdafforn]

weber DCOE 151 have bypass screws that allow balancing of adjacent chokes..
Not sure about the ones on your twinc
Tim

[gjz30075]

You can twist the throttle shaft, as previously mentioned, if the shaft is brass, as the early ones are. If they're black (steel) then maybe the throttle plates are equally settled in the bores, which requires removal of the carb to fix.

Greg Z
'72 Sprint

[bill308]

Hi Bruce.

The carbtune certainly shows the number 4 cylinder more closed (higher vacuum) than the number 3 cylinder.

I've performed the bend the shaft fix a couple of times. It did make things better, but I found it really hard to get just the right amout of bend. The difficulty is getting a good grip on the ends of the shaft and then it's still a trial and error approach. Even when you finally get it just right, over time, the shaft relaxes and some of the twist returns. If you're really good, you can over twsit the staft a little then when it relaxes again it will hold for a good long time. The problem is that the shaft return spring is on the end of the shaft away from the actuating lever, so the shaft is always under torsion. I always thought it would be good if the return spring could be relocated to the lever side to avoid this problem but never tried to do this. Another possibility is to completely remove the rotational return spring from the shaft(s) and fit a stronger (or helper) spring to the lever. This would remove the torsional load but might create some other issues.

I hope this helps.

Bill

[Dave-M]

Or remove the carb and ensure both plates are fitted correctly. This is not an easy job but if they have been removed in the past it's almost certain that incorrect re-fitting is causing the imbalance.
If you remove the trumpets, aux venturi and chokes, slacken the idle speed screw off and hold the carb body up to a light source I will bet you can see why they are out of balange.
The throttle plate with the larger peripheral gap needs its fixing screws slackening and by shuffling it to and fro a small amount you should achieve an equal radial gap or even no gap at all, when held up to the light.
The spindle should not be twisted and stronger springs are a bodge.
If you are convinced it's the spindle just replace it.
Regards
Dave

[Bruce Crowthorne]

Thanks guys.
I think I'll take the offending carb off and check if the throttle butterflies are centred and if there is a significant gap on number 3 choke.
I had partially stripped the carbs but had stopped short of taking the spindles out - but maybe a previous owner has upset a butterfly.

If correcting the centring of the butterfly doesn't fix it then maybe twisting the spindle might come into play....
Bruce

[types26/36]

then maybe twisting the spindle might come into play....Bruce

I tried that once.....after centrlizing the butterfly and checking the gap to the body,also looking where the butterflys were in relation to the progression holes.........it did'nt work for me, ended up pitting an other carb on......good luck

[tdafforn]

I believe another method is to use some V. fine drills and drill holes in the butterflies
Tim
It is probably worth going to Keith's newsgroup on yahoo devoted to twin choke carbs

[Foxie]

weber DCOE 151 have bypass screws that allow balancing of adjacent chokes..

Tim

Tim,

Very coincidently, I just started up after a rebuild today with a new set of Weber 45s (Type 152), made in Spain. There is serious unbalance between the chokes on both carbs. I am curious about the DCOe 151 bypass screws you mention. On the 45s there is a set screw and lock nut on the upper outside on each carb. Is this it ? Any idea how it is set ?

Sean Murray

[bill308]

Sean,

I believe the 151's do have an air bypass circuit. If this is the case, in an ideal world, both air bypass valves would be seated, no bypass required. If your bypass measurements show one is flowing more than the other, open the bypass circuit on the lower flowing (higher vacuum) cylinder and equalize them. This is a nice feature which I have on both the DHLA E's for the Lotus and the DCNF's that are fitted to my 308.

Bill

[bill308]

tdaforn,

Yes, drilling holes in the throttle plate can achieve the same results as an air bypass circuit. If I were to do this, I would drill the hole away from where the transition holes are. I suppose it is reversible in that one could always solder up the hole and redrill as necessary. Somehow, though, I'm not a fan of this procedure, I've never tried it, but maybe that's my porblem.

Bill

[lotuselan2]

Bruce
Have you checked that the imbalance is at larger throttle openings? It may just be at idle or idle plus a bit. Bending throttle shafts, the likely cause and solution, will affect balance at all openings.

If only at idle, working on progression holes might be a better fix.
Ken

[bill308]

I'm not a big fan of Keith with regard to his advice. He seems to be a very bright fellow but his views are rather radical IMO and I've had a couple of shoot outs with him over carb issues in postings. I always got the impression that Keith thought Webers were poorly engineered. I always thought they were pretty good and only exceeded by Dellortos in overall performance among all carbs. His discussions of two phase flow, presumabley air plus vaporixed fuel, always left me with the feeling that I was being snowed. It may be that he knows a lot more about carburation than I and my friend Tim, also a degreed ME, but I'm not sure.

Webers, when properly set up, are pretty good.

Bill

[Foxie]

Sean,

If your bypass measurements show one is flowing more than the other, open the bypass circuit on the lower flowing (higher vacuum) cylinder and equalize them.
Bill

Thanks Bill, I'll try that in the morning. I haven't measured the flows except by ear, but cyls no. 1 and 4 don't come in on idle at all.

Sean